1. Field of Invention
This invention relates to a method and system for the drift correction of spectrum images. More particularly, this invention describes a method and system for drift correction of spectrum images using concurrent collection signals, for example, both electron and x-ray signals, while eliminating periodic correction-image collection and correction steps.
2. Discussion of Prior Art
Spectrum Imaging is the collection and spatial registration of all spectral events, yielding a spectral data cube. Spectral Events are the converted x-ray energies from an x-ray detector/pulse processor, the value of which is proportional to the energy of the x-ray. Various analytical methods can be applied to the spectral data cube, ranging from simple elemental region-of-interest images, to spectral summation of the pixel elemental weight percent, to true chemical phase images. By the term “region-of-interest images” is meant a region defined with regard to a span of x-ray energies that corresponds to peak location of an element in an x-ray spectrum. The sum of x-ray counts over the defined energy region is collected for each pixel, creating an element image. By the term “spectral summation of the pixel elemental weight percent” is meant the summation of the x-ray spectra that correspond to pixels inside a spatially defined region of interest. This resultant x-ray spectrum can then be quantified to yield the weight percent values of the constituent elemental distribution. By the term “true chemical phase images” is meant the processing of the spectral data cube by methods such as principal components or multi-variant statistical analysis, both of which use statistical methods to transform the data into a basis where it can be visualized according to an eigenvector formulation.
Collecting a spectrum image involves scan generation which is the process of generating an x-y spatial raster scan using a scanned excitation source (electron, ion or photon beam) or a stationary excitation source and scanning specimen stage. Typically scan generation is used to collect an image from any signal, where the source of the signal is converted to analog or digital from either a backscattered or secondary electron detector, but it can be a signal from any detector connected to a microscope (e.g., absorbed current, EBIC, or cathodoluminescence detectors). With traditional spectrum imaging, the signal source is only the converted x-ray energies.
The time required to collect a spectrum image is dependent on the x-ray photon flux, the amount of x-ray dwell time per pixel, the image size and the number of image frames scanned. With typical x-ray detector/pulse processors configured for adequate energy resolution, this time is on the order of 15 minutes to several hours. During this time, the specimen can drift due to stage movement or instrument electronics. This drift is detrimental to the collection of a spectrum image as it results in a spatial smearing of features.
There are two methods in the prior art known for drift correction. Both methods entail stopping the collection of the spectrum image to collect a drift correction-image using an electron imaging source (secondary or backscattered).
In the first method (Lamvik, M. K, 1989), the spectrum image is collected sequentially (Ingram et al., 1988, Hunt and Williams, 1991), that is, pixel by pixel. At every n pixels, a correction-image is collected and saved. After the entire spectrum image is collected and saved, the correction-images are compared to a reference-image and drift correction vectors computed. These correction vectors are then applied to the saved spectrum image and the x, y pixels are re-packed to new locations. This procedure is known as passive drift correction and is not widely in use due to the disadvantage of requiring long collection times per pixel in collecting the sequential spectrum image.
In the second method, the spectrum image is collected using the method of position-tagged spectrometry (Legge and Hammond, 1979, Mott et al., 1995) in which the x-rays are tagged with the position of origin while the pixels are continually scanned. At every n frames, a correction-image is collected and compared to a reference-image and drift correction vectors computed. The drift correction vectors are used to adjust the next x, y scan position. This method is known as active drift correction, and must be applied during collection. It has the disadvantage of requiring that the collected image area be a sub-region of the available field of view. This sub-region is allowed to move around as the procedure tracks drift. The application of drift correction vectors is typically preformed using additional analog offset circuits or electron gun tilt adjustments and as such must be calibrated for proper operation. The largest disadvantage is that the drift correction vectors are applied after drift has been detected. This means that, between drift correction-images, x-ray events can be inserted in incorrect x, y pixel locations. Depending on the drift rate, the spectrum image can still have spatial smearing of features despite the use of an active drift correction procedure.